chemical passivation of metallic coatings on steel sheet · characteristics of metallic coatings...

Publication 130 – E

Chemical Passivation of Metallic Coatings on Steel Sheet

Stahl-Informations-Zentrum

130-E Chemical Passivation of Metall_010910_130 15.09.10 11:02 Seite 1

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Stahl-Informations-Zentrum

Stahl-Informations-Zentrum is anorganisation of steel producing andprocessing companies in Germany. It provides company-neutral, market-and application-oriented informationon the use and processing of steel.

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Publishing information

Publication 130 – E“Chemical Passivation of MetallicCoatings on Steel Sheet”Edition 2010ISSN 0175-2006

Publisher:Stahl-Informations-ZentrumP.O. Box 10 48 42 40039 DüsseldorfGermany

Author/Editor:The information on which this

publication is based was researchedand compiled with utmost care, relying on input from Stahl-Informa-tion-Zentrum's member companies.However, no liability can be acceptedfor the accuracy of the informationpresented herein.

Reprint of this publication, or ofany part hereof, is permitted onlywith the publisher's written approvaland subject to clear identification ofthe source.


Member of the Stahl-Informations-Zentrum: • AG der Dillinger Hüttenwerke• ArcelorMittal Bremen GmbH• ArcelorMittal Commercial RPS Sàrl• ArcelorMittal Duisburg GmbH• ArcelorMittal Eisenhüttenstadt GmbH• Benteler Steel Tube GmbH• Gebr. Meiser GmbH• Georgsmarienhütte GmbH• Rasselstein GmbH• Remscheider Walz- und Hammerwerke Böllinghaus

GmbH & Co. KG• Saarstahl AG• Salzgitter AG• ThyssenKrupp Electrical Steel GmbH• ThyssenKrupp GfT Bautechnik GmbH• ThyssenKrupp Steel Europe AG• ThyssenKrupp VDM GmbH• Wickeder Westfalenstahl GmbH

ContentsPage

1 Introduction ............................. 32 Corrosion protection

characteristics ofmetallic coatings ....................... 3

3 Temporary protection during storage and shipping ................................... 4

4 Application of chemicalpassivating agents ..................... 5

5 Processing of chemicallypassivated flat products ............ 5

6 Conclusions .............................. 67 Standards and regulatory

instruments .............................. 78 Further publications ................. 7


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1 Introduction

Passivating agents for a temporaryprotection of metallic coatings (hot-dip coatings, electrolytic zinc coat-ings) were formerly based on chrome(VI)-containing chemicals, also knownas chromates. The use of these com-pounds has been severely restrictedor even prohibited by the introduc-tion of legislation addressing variousindustries. The substance bans im-posed to date (March 2009) apply tomaterials for electronic and electricalconsumer goods, passenger cars andcommercial vehicles with weightsnot exceeding 3.5 tonnes. For Ger-many, the relevant regulations andexceptions are laid down in theElectrical and Electronic EquipmentAct based on the “RoHS” Directive2002/95/EC and in the End-of-LifeVehicles Act based on the “ELV” Directive 2000/53/EC.

In branches of industry exemptfrom such regulations, the use ofcoatings passivated with Cr(VI) con-taining agents is still an option. How-ever, in the interests of labour safetyin steelmaking and steel processingand for environmental reasons, steel-makers are endeavouring to limit theuse of Cr(VI) containing chemicalseven further, up to the point wherethey can be eliminated altogether.

The present publication is aimedat processors and users of the above-mentioned chemically passivated flatproducts and goods manufacturedtherefrom. In order to ensure thathot-dip coated and electrogalvanizedproducts with chemical passivationcan still be marketed in an altered le-gal situation, substitutes for Cr(VI)-containing passivating agents havebeen developed. Given their chemi-cal composition and action princi-ple, these substances tend to resultin appreciably different applicationcharacteristics in downstream pro-cessing.

Since research and developmentof these substitutes is ongoing, theirproperties cannot be described indetail in this publication. However,they continue to provide a temporaryprotection of surfaces against cor -rosion.

2 Corrosion protection, characteristics of metalliccoatingsMetallic coatings improve the

application characteristics of steelsheet by enhancing its corrosion re-sistance. In selecting the optimalcoating for a given purpose, it is nec-essary to consider the subsequentloading and exposure profile.

Depending on the alloy compo-sition, metallic coatings exhibit dif-ferent corrosion-inhibiting mecha-nisms. As a general rule, a higheraluminium content tends to diminishthe cathodic protection effect whileenhancing the barrier properties ofthe metallic coating.

Accordingly, coatings affordinga high level of cathodic protectionattain their maximum effectiveness

when covered with a sealing layer.Metallic coatings with a high barriereffect, especially aluminium-zinc (AZ)and aluminium-silicon (AS) coatings,exhibit very good anti-corrosion prop-erties even when left bare. A generalevaluation of the corrosion charac-teristics of various coatings is givenin Table 1.

When the unprotected surfaceof a zinc-containing metallic coatingis brought into contact with water,zinc hydroxide forms as a reactionproduct. If the exposure to moistureremains brief and the surface can dryfully afterwards, this compound re-acts into basic zinc carbonate withCO2 from the ambient air. The basiczinc carbonate forms a deposit onthe surface which acts as a passivesealing layer. However, this naturalpassivation effect takes much timeto develop and is quite susceptibleto disturbances. The most frequentdisturbance is the accumulation ofmoisture due to inadequate ventila-tion, e.g., in gaps and cavities. Thewater then reacts with zinc from thesurface coating, with large amountsof zinc hydroxide (white rust) form-ing in the process, Fig. 1. The forma-tion of white rust is irreversible anddestroys the metallic coating. High-


Corrosion behaviour Z ZA AZ AS

Undeformed surface O + ++ ++

Bending shoulder O + + +

Cut edge O O – –

Table 1: Corrosion propertiesof hot-dip coatings

O = standardThis table gives a general evaluation reflecting today's level of experience. It does not apply to all application situations. In cases of doubt the vendor should be contacted.

Water

Zinccoating

Steel sheet

White rustZn Zn2+ + 2e-

Zn2+ + 2 OH- Zn(OH)2

H2O + 1/2 O2 + 2e- 2 OH-

Fig. 1: Formation of white rust due to direct exposure of zinc coating to water


The actual protection duration de-pends mainly on atmospheric, ship-ping and storage conditions.

Common methods of protectingthe surface against moisture are chem-ical passivation and oiling, the pro-tective effect of which is illustratedin Fig. 2. In the latter treatment, avariety of user-specific oil gradesare employed (see also Publication127 – E). The quantity, type and gradeof oil can be agreed upon with thesteelmaker and must be stated in thepurchase order.

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aluminium coatings are more passivedue to the presence of aluminiumoxide on the surface, but are likewisedestroyed by lingering moisture.

3 Temporary protection during storage and shippingCathodically active metallic coat-

ings are sensitive to moisture andmust therefore be protected (for fur-ther information refer to Publication114). Effective from its ex-works delivery date, application of a tem-porary protection layer will keepthe product corrosion-free for threemonths. Provided that it is properlypacked, shipped, loaded and stored.

In chemical passivation, the exte-rior of the metallic coating is treatedwith substances which initiate a con-version of the highly reactive metalsurface and result in the formationof a passivating conversion layer oflittle reactive metal compounds. Thislayer acts as a barrier and protectsthe underlying coating metal fromdirect corrosion attack by moisture,Fig. 3. The protection is of limitedduration, as with an oil film, and depends essentially on ambient con-ditions.

Generally, with all surface pro-tection methods discussed here, itshould be noted that a risk of corro-sion damage caused by condensedwater can never be ruled out. In win -ter, an improved corrosion resistancecan be obtained by ordering suitablypacked material.

In the case of unpackaged coilsor panels, an abrupt temperaturechange from cold to warm generallycauses dew to form on surfaces ex-posed to an airflow, and even in theair gap between coil windings. There,moisture will be retained for very longperiods as a result of capillary forces.

On coils fully enclosed in film orpaper, condensation will settle most-ly on the packaging. However, pene-tration of moisture into the wrappingcannot be fully prevented. Therefore,packaged coils should always beunpacked as soon as the coil temper-ature equals that of the ambient airso that any moisture which has penetrated through the packaging canevaporate. At least two days shouldbe allowed for thermal equalization.Every time the product is placed orshifted in storage at its destination,the potential condensation risksshould be examined, Fig. 4.

If moisture has condensed on acoil despite all precautions (e.g.,because of a large temperature dif -ference or high humidity in thewarehouse), the condensed dropletswill form a continuous water filmwhich penetrates deep into the coilwindings as a result of capillaryforces. To avoid irreparable whiterust damage, the coil must then beprocessed immediately.


Oil film

Zinccoating

Steel sheet

Water

Fig. 2: Protection based on application of an oil film

Passivatinglayer

Zinccoating

Steel sheet

Water

Fig. 3: Barrier effect of the passivating layer

Natural passivation by formation of zinc carbonateInitial formation of zinc hydroxide:Zn + H2O + 1/2 O2 ⇒ Zn(OH)2

Drying/ventilation: Conversion into zinc carbonate5 Zn(OH)2 + 2 CO2 ⇒ Zn5(OH)6 (CO3)2 + 2 H2O

Formation of white rustContinuous, in the absence of drying:Zn + H2O + 1/2 O2 ⇒ Zn(OH)2


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Apart from condensed moisture,soluble salts have a highly detrimentaleffect on the coating. Sodium chlo-ride – as a constituent of road salt orsea salt – accelerates the corrosionprocess many times over. It is there-fore recommended to use exportpackaging where goods are loadedand shipped (whether by truck, rail-way or inland waterway transport)in the winter or near the sea, andthe cargo-carrying vehicle should betightly sealed.

4 Application of chemicalpassivating agentsOn continuous coil coating lines,

the passivating medium is applied tothe metal coating “in line”, i.e., at fullprocess speed (up to 250 m/min) inthe processing train. This involvesthe physical application, quantity

control and, finally, drying of thepassivating agent before the productexits the line, Fig. 5.

Application is by the roll method,in the spray-squeeze variant, or byChemcoater. Drying is usually per-formed with hot air or, in individualcases, using NIR heater.

The type and quantity of passi-vating agent applied is adapted by thesteelmaker so as to ensure a suffi-cient degree of temporary protection.Given the high process speeds in-volved, passivating agent splatter,streaks or stain formation cannotalways be avoided. This fact is reflect-ed in the relevant quality standards.However, such discolorations merelyconstitute localized visual imperfec-tions which do not impair quality.

5 Processing of chemicallypassivated flat productsIn processing steel sheet with a

chemically passivated metal coating,interactions with the processing operation may occur. Mechanical operations (slitting, cutting to length,shaping or joining) will alter ordestroy the passivation layer in thecontact area. Exposure to auxiliaryprocess fluids such as metalworkingfluids or cleaning products has thesame effect. Processors should there-fore expect that the original corro-sion protection will be lost, or atleast markedly reduced, after suchoperations.

Conversely, passivating agentresidue may be carried over, by me-chanical or chemical abrasion, ontotools, equipment or process media,thus impairing their operability.


– 5

0

5

10

15

20

25

Tem

pera

ture

[ºC]

Relative humidity [%]40 50 60 70 100

Condensationrange

1

2

Fig. 4: Diagram for determining the condensation risk

Condition example 1:Temperature of sheet stack: +8 °CStorage room temperature: +21 °CRelative humidity in storage room: 55%

Interpretation:From the point where an imaginary horizontal line through +21 °C intersects the 55% curve, proceed vertically downwards to the horizontal line passing through +8 °C. The condensation range is intersected.

A condensation risk exists.Material placed in storage in a cold state must not be unpacked until temperature equalization has taken place.

Condition example 2:Storage room temperature: +19 °CRelative air humidity in storage room: 50%

Interpretation:From the point where an imaginary horizontal line through +19 °C intersects the 50% curve, proceed vertically downwards to the boundary of the condensation range.The level of the tangential point indicates the minimum acceptable steel temperature (+10.5 °C)

There is no risk of moisture condensing if the steel temperature exceeds a value of approx. +10 °C.


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To minimize or avoid this effect,care should be taken to ensurethat the following conditions aremet where aqueous auxiliary processfluids are used:a) The pH value of the solution or

emulsion should be neutral orslightly alkaline; at pH levels > 8the passivating film may be at-tacked.

b) The medium should be preparedwith deionized water to preventprecipitation of insoluble prod-ucts of reaction with constituentsof the passivating agent.

In general, the process capabilityof chemically passivated productwith surface-sensitive downstreamoperations should be examined be-forehand.

This is particularly true for phos-phating, painting and adhesive bond-ing processes. For adhesive bonding,good adhesion is critical. The above -mentioned operations typically re-quire a narrowly defined surfacecondition to produce the desired re-sult. Restoring to a defined surfacecondition will usually be difficult after chemical passivation. As a rule,this can only be achieved by removalof the entire passivating layer andactivation of the metallic surface.

The experience gained fromtests with Cr(VI)-containing passivat-ing treatments can typically not beapplied to Cr(VI)-free substitutessince the latter may have an entirelydifferent chemical composition.

Where chemical passivation isincompatible with a given down-stream process, it is recommendedto use oiled material instead.

6 Conclusions

For steelmakers and steel users,the transition from Cr(VI)-containingto Cr(VI)-free passivating methodsconstitutes a further step forward onthe path towards sustainable and re-sponsible use of surface-coated steelproducts. The questions arising inthe context of this rapid changeoveralso provide an opportunity to reviewone’s own processing routines. In thesteelmaking companies, developmentand process departments are readyto assist customers in resolving theseissues and in the subsequent opti-mizing.


Fig. 5: Hot-dip metal coating line with passivating unit

Passivating unit withhot-air dryer

Direction of strip movement


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7 Standards and regulatoryinstruments

EN 1623Cold rolled strip and sheet – Technical delivery conditions – General structural steels

EN 10338Hot rolled and cold rolled non-coatedflat products of multiphase steels forcold forming – Technical deliveryconditions

EN 10152Electrolytically zinc coated cold rolledsteel flat products for cold forming – Technical delivery conditions

EN 10346Continuously hot-dip coated steel flatproducts – Technical delivery con -ditions

RoHS Directive 2002/95 ECRestriction of the use of Certain Hazardous Substances in Electricaland Electronic Equipment 2000/53/EC“ELV”)

ELV Directive 2000/53/ECend-of life vehicles

8 Further publications

For more information on this subject,individual copies of the followingpublications are available from Stahl-Informations-Zentrum free of charge:

Charakteristische Merkmale 092Elektrolytisch verzinktes Band undBlech

Charakteristische Merkmale 095Schmelztauchveredeltes Band undBlech

Merkblatt 109Stahlsorten für oberflächenveredeltesFeinblech

Merkblatt 114Verpackung, Lagerung und Transportvon unbeschichtetem und beschich -tetem Band und Blech

Publication 122 – EWeldable Corrosion-Protection Prim -er – Thin Film-Coated Steel Sheets forthe Automotive Industry

Publication 127 – ELubrication of sheet strip and panels



Stahl-Informations-Zentrumim Stahl-ZentrumP.O. Box 10 48 42 · 40039 Düsseldorf, GermanySohnstrasse 65 · 40237 Düsseldorf, GermanyE-mail: [email protected] · www.stahl-info.de


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